The war for signals intelligence was as much about code-making as it was about codebreaking, as illustrated by one famous and tragic story. On the morning of December 7, 1941, George Marshall, the Army’s chief of staff, had an important message to send to his Pacific Command: Japan had decided that it could no longer mediate its differences with the United States through politics; war was likely. But how to transmit this information? The only system available to the nation’s top military and political leaders had long been considered insecure. The message was sent instead by the comparatively
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